Guillain-Barré syndrome is thought to be caused by a problem with the immune system, the body's natural defence against illness and infection.
Normally the immune system attacks any germs that get into the body. But in people with Guillain-Barré syndrome, something goes wrong and it mistakenly attacks the nerves.
This damages the nerves and stops signals from the brain travelling along them properly, which can cause problems such as numbness, weakness and pain in the limbs.
It's not clear exactly why this happens. The condition is not passed from person to person and is not inherited.
Sometimes Guillain-Barré syndrome appears to have a particular trigger. These are some of the main triggers.
In most cases, Guillain-Barré syndrome occurs a few days or weeks after an infection.
Infections that have been known to trigger the condition include:
In the past, vaccinations (particularly the flu vaccine used in the US during a swine flu outbreak in 1976) were linked to an increased risk of Guillain-Barré syndrome.
But research has since found the chances of developing the condition after having a vaccination are extremely small.
For example, a study into the vaccine used during the 2009 swine flu outbreak found that for every million people who had the vaccination, there were fewer than 2 extra cases of Guillain-Barré syndrome.
And evidence suggests that you are far more likely to get Guillain-Barré syndrome from an infection, such as the flu, than the vaccine designed to prevent the infection, such as the flu jab.